The present invention relates generally to fabrication of field effect transistors having scaled-down dimensions, and more particularly, to fabrication of a field effect transistor having dual gates in SOI (semiconductor on insulator) technology, for minimizing short-channel effects in the field effect transistor.
Referring to FIG. 1, a common component of a monolithic IC is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 100 which is fabricated within a semiconductor substrate 102. The scaled down MOSFET 100 having submicron or nanometer dimensions includes a drain extension junction 104 and a source extension junction 106 formed within an active device area 126 of the semiconductor substrate 102. The drain extension junction 104 and the source extension junction 106 are shallow junctions to minimize short-channel effects in the MOSFET 100 having submicron or nanometer dimensions, as known to one of ordinary skill in the art of integrated circuit fabrication.
The MOSFET 100 further includes a drain contact junction 108 with a drain silicide 110 for providing contact to the drain of the MOSFET 100 and includes a source contact junction 112 with a source silicide 114 for providing contact to the source of the MOSFET 100. The drain contact junction 108 and the source contact junction 112 are fabricated as deeper junctions such that a relatively large size of the drain silicide 110 and the source silicide 114 respectively may be fabricated therein to provide low resistance contact to the drain and the source respectively of the MOSFET 100.
The MOSFET 100 further includes a gate dielectric 116 and a gate electrode 118 which may be comprised of polysilicon. A gate silicide 120 is formed on the polysilicon gate electrode 118 for providing contact to the gate of the MOSFET 100. The MOSFET 100 is electrically isolated from other integrated circuit devices within the semiconductor substrate 102 by shallow trench isolation structures 121. The shallow trench isolation structures 121 define the active device area 126, within the semiconductor substrate 102, where a MOSFET is fabricated therein.
The MOSFET 100 also includes a spacer 122 disposed on the sidewalls of the gate electrode 118 and the gate dielectric 116. When the spacer 122 is comprised of silicon nitride (Si3N4), then a spacer liner oxide 124 is deposited as a buffer layer between the spacer 122 and the sidewalls of the gate electrode 118 and the gate dielectric 116.
A long-recognized important objective in the constant advancement of monolithic IC (Integrated Circuit) technology is the scaling-down of IC dimensions. Such scaling-down of IC dimensions reduces area capacitance and is critical to obtaining higher speed performance of integrated circuits. Moreover, reducing the area of an IC die leads to higher yield in IC fabrication. Such advantages are a driving force to constantly scale down IC dimensions.
As the dimensions of the MOSFET 100 are scaled down further, the junction capacitances formed by the drain and source extension junctions 104 and 106 and by the drain and source contact junctions 108 and 112 may limit the speed performance of the MOSFET 100. Thus, referring to FIG. 2, a MOSFET 150 is formed with SOI (semiconductor on insulator) technology. In that case, a layer of buried insulating material 152 is formed on the semiconductor substrate 102, and a layer of semiconductor material 154 is formed on the layer of buried insulating material 152. A drain 156 and a source 158 of the MOSFET 150 are formed in the layer of semiconductor material 154. Elements such as the gate dielectric 116 and the gate electrode 118 having the same reference number in FIGS. 1 and 2 refer to elements having similar structure and function. Processes for formation of such elements 116, 118, 152, 154, 156, and 158 of the MOSFET 150 are known to one of ordinary skill in the art of integrated circuit fabrication.
In FIG. 2, the drain 156 and the source 158 are formed to extend down to contact the layer of buried insulating material 152. Thus, because the drain 156, the source 158, and a channel region 160 of the MOSFET 150 do not form a junction with the semiconductor substrate 102, junction capacitance is minimized for the MOSFET 150 to enhance the speed performance of the MOSFET 150 formed with SOI (semiconductor on insulator) technology.
In addition, referring to FIGS. 1 and 2, as the dimensions of the MOSFETs 100 and 150 are scaled down further, the occurrence of undesired short-channel effects increases, as known to one of ordinary skill in the art of integrated circuit fabrication. With short-channel effects, the threshold voltage of the MOSFET changes such that electrical characteristics of such a MOSFET become uncontrollable, as known to one of ordinary skill in the art of integrated circuit fabrication. In the prior art MOSFETs 100 and 150 of FIGS. 1 and 2, the gate dielectric 116 and the gate electrode 118 are formed on one surface of the channel region of the MOSFET. However, for controlling the electrical characteristics of the MOSFET, forming a gate dielectric and a gate electrode on a plurality of surfaces of the channel region of the MOSFET is desired to minimize undesired short channel effects.
Accordingly, in a general aspect of the present invention, a field effect transistor is fabricated to have dual gates on two surfaces of the channel region of the field effect transistor formed in SOI (semiconductor on insulator) technology, to minimize undesired short channel effects.
In one embodiment of the present invention, in a method for fabricating a field effect transistor having dual gates, on a buried insulating layer in SOI (semiconductor on insulator) technology, a first layer of first semiconductor material is deposited on the buried insulating material. The first layer of first semiconductor material is patterned to form a first semiconductor island having a first top surface and a second semiconductor island having a second top surface. The first and second semiconductor islands are comprised of the first semiconductor material. An insulating material is deposited to surround the first and second semiconductor islands, and the insulating material is polished down until the first and second top surfaces of the first and second semiconductor islands are exposed such that sidewalls of the first and second semiconductor islands are surrounded by the insulating material.
In addition, a gate dopant is implanted into the second semiconductor island. A layer of back gate dielectric material is deposited on the first and second top surfaces of the first and second semiconductor islands. An opening is patterned through the layer of back gate dielectric material above the first semiconductor island such that a bottom wall of the opening is formed by the first top surface of the first semiconductor island. A second layer of second semiconductor material is grown from the exposed first top surface of the first semiconductor island and onto the layer of back gate dielectric material. A front gate dielectric is formed over a portion of the second layer of second semiconductor material disposed over the second semiconductor island. A front gate electrode is formed over the front gate dielectric. The second semiconductor island forms a back gate electrode, and a portion of the layer of back gate dielectric material under the front gate dielectric forms a back gate dielectric.
The present invention may be used to particular advantage when the first semiconductor material forming the first and second semiconductor islands are comprised of silicon and when the second layer of second semiconductor material is silicon epitaxially grown from the top surface of the first semiconductor island through the opening in the layer of back gate dielectric material.
In this manner, the back gate dielectric and the back gate electrode are formed on a bottom surface of the channel region of the field effect transistor, and the front gate dielectric and the front gate electrode are formed on a top surface of the channel region of the field effect transistor. With formation of such gate dielectrics and gate electrodes on a plurality of surfaces of the channel region of the field effect transistor, electrical characteristics of the field effect transistor are better controlled to minimized undesired short channel effects. In addition, because the field effect transistor is formed in SOI (semiconductor on insulator) technology, junction capacitance is minimized to enhance the speed performance of the field effect transistor.